This invention is concerned with the preparation of glass by the fusion of a number of particulate raw materials. It is directed to the use of concentrated alkali metal silicate solutions to form hardened agglomerate with the highly alkaline batch used to prepare soluble alkali metal silicate glass.
Silicate glasses and in particular soluble silicate glasses, are prepared by fusion of sand with a source of alkali metal, generally sodium carbonate. Sand and sodium carbonate are available as fine particulate material which must be conveyed, blended and fed to the furnace in which the fusion is carried out. The fine particle content and other physical properties of these materials lead to problems during the handling and fusion steps. These problems include dusting, segregating or demixing, loss of raw materials and air pollution. These problems persist into the furnace resulting in an inhomogeneous product, attack on the refractories and air pollution, among other problems. The finely divided nature of the batch also contributes to less efficient heat transfer to the batch.
It has long been recognized that some method of pelletizing, briquetting or agglomerating after complete and intimate mixing of the batch would be desirable. The batch required for preparing insoluble glass contains a relatively limited amount of alkali and can be moistened with water to form damp agglomerates or to aid in forming pellets or briquettes using pressure. These materials must be dried before storage and/or use as furnace feed thereby requiring extra steps and expense. The more desirable method of simply forming a damp furnace feed requires 2 steps, additional equipment and expense. This method involves adding water (.about.1% of the batch weight) to the raw materials at the mixing point to prevent dusting, and then adding 2 to 4% additional water just prior to charging the furnace. These methods provide some relief from the aforementioned problems to the insoluble glass industry.
Preparation and use of the alkali rich raw material batch required to produce soluble silicate glass, entails the previously discussed problems, plus the additional complications of a large proportion of sodium carbonate. This large proportion of carbonate renders the use of water to agglomerate the batch as difficult at best. It is necessary to add about 4.5% water or dilute caustic solution to provide the tacky condition required for efficient furnace feed. Additionally, the batch must be maintained above 35.degree. C to retain any benefit, below this temperature sodium carbonate forms higher hydrates and quickly consumes free moisture coating the batch particles. The formation of these higher hydrates introduce a cementing action, movement of the batch is impeded and it is easy to have the entire batch harden into a cohesive mass.
Handling a damp batch of raw materials with a high proportion of alkali poses many problems and requires special handling equipment. It is an object of this invention to provide free-flowing, storable hard agglomerates of soluble glass forming raw materials without the need for a drying step or the use of pressure. It is also an object of this invention to provide a process for preparing such free-flowing agglomerated batches without maintaining an increased temperature after wetting. It is a further object of this invention to provide a one step process whereby the agglomeration takes place during or immediately after the batch mixing step.